Anti-shock hydraulic circuit



Feb. 12, 1957 G. F. RoHRscHElB ANTI-SHOCK HYDRAULIC CIRCUIT l Filed oct. 14, 1953 2 Sheets-Sheet l G. F. ROHRSCHEIB ANTI-SHOCK HYDRAULIC CIRCUIT Feb. 12, 1957 2 Sheets-Shea?l 2 Filed OGL. 14, 1953 www ANTI-SHOCK HYDRAULIC CIRCUIT George F. Rohrscheib, Pewaukee, Wis., assignor to Verson Allsteel Press Company, Chicago, Ill., a corporation of Delaware Application October 14, 1953, Serial No. 386,096

13 Claims. (Cl. titl- 52) The present invention relates to a hydraulically operated metal forming apparatus, and particularly to a new and improved hydraulic circuit which is substantially shock free in all phases of its operation.

Hydraulic press circuits are operated by a hydraulic iluid such as oil, which is almost incompressible. To obtain work from this fluid or oil its volume in a closed space, such as a press cylinder or the like, must be increased approximately one-half of one percent for each desired thousand pounds per square inch increase of pressure. This compressed oil takes on an explosive characteristic when released too suddenly, and the resulting shock is detrimental to equipment and piping of hydraulically operated machines such as metal working presses. The release of the compressed oil is called decompression and constitutes a serious problem for which no Wholly satisfactory solution has heretofore been found.

An early method of controlling hydraulically operated machinery was by means of one-way or unidirectional pumps, and a four-way valve. The function of the valve was to direct the flow of the hydraulic Huid or oil to obtain the desired action from the machine. When so used these valves were called reversing valves, as the name exactly described their function. Much time and elort has been expended in the design of such valves, but oil decompression was, and remains, difficult to obtain without undue shock. And obviously the faster the reversal was eected, the greater the shock.

Later a pump was developed which delivered oil from either of two ports without reversing the direction of rotation of the pump parts, and the four-Way valve could be eliminated because oil flow direction in the system could be controlled solely by means of the pump. Further refinements in the pump permitted oil decompression to take place Within the pump during the period of machine reversal. This method of decompression was quite satisfactory from the standpoint of shock elimination, but it took considerable time to effect decompression.

The present invention is concerned with a novel hydraulic circuit for hydraulically operated machinery such as a metal working press, wherein oil or hydraulic fluid decompression within an enclosed space such as the work cylinder of a press, is effected without appreciable shock to any of the machinery parts, and is effected substantially instantaneously, thus obviating any time interval for bleeding oit the high pressure lluid through a restricted valve port, pump port, or the like.

The present invention is illustrated and described with reference to a rubber pad metal forming press wherein the lluid under pressure is conned within an inatable rubber bag or sack of substantial dimension, which is inflatable within delined limits so that maximum pressing force may be applied to the part being formed. The hydraulic circuit forming the subject matter of this invention also evacuates the inflatable bag or enclosed space of hydraulic fluid or oil, and places it under a partial vacuum when the work is moved into and from forming position.

It is a primary object, therefore, of the present invennited States Patent Ol 2,780,918 Patented Feb. 12, 1957 tion to provide a new and improved substantially shockfree hydraulic circuit for hydraulically operated machinery, wherein the decompression of oil or hydraulic 'luid confined under a high pressure is effected substantially instantaneously.

Another object is to provide a new and improved antishock hydraulic press circuit wherein uid under pressure is supplied to the enclosed work space of the press and withdrawn therefrom by a pair of pumping means, one of which supplies uid at high pressure and the other of which supplies uid at high volume and low pressure, and rapid controlled decompression of uid conned under high pressure in the Work space is elected through the high pressure pumping means.

` Another object is to provide a new and improved hydraulic circuit for hydraulically operated rubber pad forming presses wherein'the pressure is applied to the work from an inflatable bag, the inflatable limits of which are confined, and which may be subjected to a partial vacuum.

Another object is to provide a new and improved hydraulic press of the rubber pad forming type, wherein the pressure is applied to the work from an inatable bag, and the hydraulic fluid under pressure is applied to and withdrawn from the bag by a pair of pumps, one of which supplies iluid at a high pressure and the other of which supplies a high volume of liuid under low pressure, the latter pump, when being driven in a certain direction, applying a partial vacuum to the bag to hold it out of work engaging position to allow the work to be moved into and out of forming position.

Another object is to provide a new and improved hydraulic press having the foregoing characteristics, which is completely automatic in its operating cycle, including moving the work into and from forming position.

Another object is to provide a new and improved rubber pad forming press which is substantially smaller than conventional rubber pad forming presses having the same or as much as half the rated tonnage of the press of the present invention.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a diagrammatic view of one form of hydraulic circuit constituting the subject matter of the present invention;

Fig. 2 is a schematic view, on a relatively small scale, illustrating al rubber pad forming press with which the hydraulic circuit of the present invention is adapted to be used; and

Fig. 3 is a diagrammatic view of a modification of the hydraulic circuit of Fig. 1.

` The reference character 10 indicates generally the rubber pad forming press which includes a frame 12 having a cylindrical housing 14 mounted thereon. Work 16 is movable into and withdrawn from the cylindrical housing 14 on a movable table or tables 18. In the particular press shown, two tables 18 are provided on which the work is formed around dies 2t). A relatively thick, heavy duty rubber pad, or the like, 22 is laid over the workpieces 16 and dies 20, and is caused to flow thereabout by means of pressure applied from an inflatable means 24, such as an inflatable rubber bag which is supplied with hydraulic uid from a pair of pumps 26, 28, driven by a reversible electric motor 30. Hydraulic fluid or oil for the pumps 26 and 28 is stored in a tank or reservoir 31 mounted on the frame 12, the pumps and motor also being mounted on the frame, as well as the valves which will be described hereinafter.

The high pressure low volume pump 26 has a pair of pump ports 32 and 34 and a drain port 36, the pump port 32 being connected by a conduit 3%, a pressure reliefvalve 40, and conduit 42 with the inflatable rubber bag 24, and drain port 36 being connected to the'l reservoir 31 by conduit 37. The pressure relief valve is set to relieve pressure in the system over a predetermined value, for example, 5000 p. s. i. through a conduit 44 to the tank 31. The low pressure high volurne pump 28 has a pair of pump ports 45 and 46 through which fluid is pumped from the pump and a drain port 48. The port 45 is connected by a conduit 50, a prefill valve 52, and a conduit 54 to the conduit 42; the ports 46 and 48 are connected by conduits 47 and 49, respectively, to the reservoir 31. When both of the pumps 26 and 28 are being driven to supply uid to the bag 24, they both pump through the conduit 42. The pumps are so arranged that they pump out or suck in through the ports 32 and 34 or 45 and 46, respectively.

The prell valve 52 has a lluid passage 56 therethrough which is normally closed by a valve member 58 provided with a stem 60 and biased to closed position by a spring 62. The prell valve S2 may be opened by uid flowing from the conduit through the passage 56 to the conduit 54 from the low pressure pump 28, or itmay be opened by means of uid supplied from the high pressure pump port 34 through conduits 64 and 66 to a piston 68 which is opposed by a spring 70, the piston 68 when moved in the valve opening direction contacting the end of the valve stem and moving the valve member 58 to the open position. As will be explained more fully hereinafter, the pump 26 holds the valve 52 open when the low pressure high volume pump 28 assists in evacuating fluid from the bag 24, and when it maintains a partial vacuum on the bag.

An unloading valve 72 is connected by conduit 74 to the conduit 50 upstream of the prell valve 52, i. e., between the pump port 45 and the valve 52. A pilot connection is made to the unloading valve 72 by a conduit 76, and when the pressure in the system reaches a predetermined value, between 100 p. s. i. and 1500 p. s. i., preferably about 500 p. s. i., the unloading valve 72 is opened and the output from the low pressure pump port 45 flows through the conduits 50 and 74, valve 72, and conduit 78, to the reservoir 31 so that the low pressure pump 28 is rendered ineffective to transfer fluid and only the high pressure pump 26 supplies uid to the bag 24. Under such conditions the valve member 58 is closed by the spring 62 and high pressure on its head.

When the low pressure pump 28 is driven in the reverse or bag evacuating direction, it takes uid in through the port 4S and expels it through the port 46 to the reservoir 31. When the bag has been emptied of fluid and the motor 30 continues to drive the pump 28 in the evacuating direction, the pump 28 takes in uid through a conduit and check valve 82 to prevent its being starved of fluid. The check valve 82 opens when a predetermined partial vacuum is created such, for example, as that measured by 18 inches of mercury on a pump 28 rated at 75.5 gallons per minute.

While the pump 28 is being driven in the bag evacuating direction, the pump 26 is likewise being `driven in the same direction, since both pumps are coupled to the motor 30. After the bagv has been completely evacuated, the pump 26 is supplied with fluid from a conduit 84 leading from the tank 31 and connected to the conduit 38, and having a check valve 86 therein to prevent reverse flow, the check valve opening when the system pressure has dropped to about zero p. s. i. rIhe pump port 34 is connected to the reservoir 31 by a conduit having a check valve 87 therein and through which the major portion of the fluid is supplied when the pump 26 is driven in the bag inating direction.

The movement yof the work into and from the work position below the inflatable bag 24 on either of the tables 18, is effected by means of hydraulic motors 88L and 88R, which are supplied with operating fluid by the pump 26 through solenoid operated three position,

four-way valves 90L and 90K. Since these valves are identical, only one will be described and the two will be diterentiated from each other by the suffixes L and R to their reference characters.

Fluid under pressure is expelled from the high pressure pump port 34, conduit 64, and conduit 92 having a restriction 94 therein, the restriction creating a back pressure, for example 300 p. s. i., at the pump port 34 suflicient to open and hold open the valve 52 when necessary. The conduit 92 is connected to port 94L of the valve 90L, and when the valve spool 96L is centered, which is its-normal position, the port 94L is connected internally through the valve 90L to a port 98L. The port 98L is connected by a conduit 100 to port 94K of the valve 90R, and this port, when the valve is in its neutral position, is connected internally through the valve to a port 98R which in turn is connected by a conduit 102 to the reservoir 31. Thus, even though the pump 26 is driven in the direction to supply fluid from the port 34 to the hydraulic motors 90L and 90R, movement of the tables is not effected, as the uid is returned to the reservoir 31.

When it is desired to move a table bearing the dies and workpieces into forming position, a solenoid 104L is energized to shift the valve spool 96L to connect the port 94L to a port 106L, which is connected by a conduit 108 to t-he head end of the motor cylinder SSL. At the same time the port 110L, which is connected by conduit 112 to the rod end of the motor cylinder 88L, is internally connected through the valve to the port 981s. Fluid flows into the head end of the motor cylinder 88L through the conduit 108 and out from the rod end thereof through the conduit 112 and to the tank through the conduit 100, valve 90R, and conduit 102. The system is interlocked so that it is possible to `energize only one solenoid at any one time, and at least one valve will always be in its neutral position. Furthermore, the system is so arranged that the valve solenoids cannot be energized unless the vacuum impressed upon the bag 24 is suicient to hold the latter out of the movement path of the table 18.

At the end of the work forming operation, solenoid 114L is energized and the high pressure pump 26 supplies uid to the rod end of the motor cylinder SSL thereby expelling uid from the head end of the cylinder, permitting the table to be withdrawn from the work housing 14. It is understood, of course, that limit switches are incorporated in the circuits of the solenoids 104L and 114L so that the proper movement of the work table is obtained.

A pressure relief valve 116 is connected by a conduit 118 to the junction of the conduits 64, 92, and 66, and is in turn connected to the tank 31 by conduit 120. This pressure relief valve is set to relieve pressure at a value in excess of that created by the restriction 94, for example 750 p. s. i., in order to protect the pump and system against overload during movement of the tables.

At the end of the work forming operation, and after the bag 24 has been evacuated of fluid, the low pressure pump 28, as previously described, maintains a partial vacuum on the bag 24, and until this vacuum reaches a predetermined value, for example, 18 inches of mercury, the solenoid' 114L controlling removal of the table from the housing 14 cannot be energized. This vacuum is sensed by a vacuum switch 122 which is connected through a vacuum switch valve 124 to the conduit 42, the conduit 42 being connected to the valve at port 126, and the switch being connected to the valve through conduit 128 and port 130. The connection of the ports in this Valve with each other is under the control of a spring centered valve spool 132. When a vacuum exists in the line, the spool 132 is centered and connects the port 126 to the port 130, thereby permitting the vacuum to reach the vacuum switch 122. On the other hand, when the pumps 2'6 and 28 are pumping iluid through the line 42,

it is necessary to protect the vacuum switch 122 from iluid under pressure. A pilot connection 134 is provided to connect the line 42 with a port 136 to apply uid under pressure against one end of the valve spool 132 to shift it when the system pressure reaches about 50 p. s. i. against the force of the centering springs, thereby connecting the port 126 to a plugged port 138 to cut oi communication between the conduit 42 andthe vacuum switch 122. The vacuum switch line 128 is then connected to drain or the tank 31 through a conduit 140 connected to a port 142 which is connected internally through the valve with the port 130. The opposite end of the valve spool 132 is similarly connected by a port 144 and conduit 146 to drain or the tank 31, so that disposition of huid accumulated in the valve can be made.

Reversal of the electric motor at the maximum pressure to be developed in the system is effected by either of a pair of pressure operated switches 148 which are connected to sense the pressure in the conduit 42. A safety overload switch 158 is also provided and is connected in the system shown to the conduit 54 which communicates with the conduit 42. This switch is set to operate at a higher pressure than are the switches 148.

When the press is not in use, the pumps 26 and 28 and the motor 36 will not be operated, and the work carrying tables 18 are in their withdrawn or retracted positions. The operation of the press is commenced by energizing the motor circuit which drives the pumps 26 and 28 in such direction as to take in fluid through the ports 32 and 45, respectively, and expel fluid through the ports i 34 and 46, respectively. Fluid is pumped from the high pressure pump 26 to open the prell valve 52, thereby establishing communication between the pump 28 and the bag 24 and the vacuum switch 122.

When the vacuum created reaches the predetermined value, at which the bag is held out of the movement path of the tables 18 the operator can close a circuit energizing either of the solenoids 104L or 104R to admit fluid to the head end of the respective hydraulic motor cylinders SSL or SSR. A work carrying table 18 is moved under the deflated and vacuum held bag 24, and at the end of its travel a limit switch in the motor circuit is actuated. The circuit to the energized solenoid is opened and the actuated valve centers itself. This limit switch also opens the circuit to the reversible electric motor 30 which immediately stops. However, it closes another circuit to the motor and the latter is automatically started up in opposite and bag inilating direction, in which the pumps 26 and 28 are driven to supply hydraulic uid under pressure to the bag 24, fluid pressure in the line 42 being communicated to the vacuum switch valve 124 to disconnect the conduit 42 from the conduit 128 to protect the vacuum switch 122 against damage by high pressures in the line 42. The pump 26, therefore, no longer holds the prell valve 52 open, but iluid output from the pump 28 reopens the valve 52.

As pressure in the line 54, 42 builds up, the bag is rapidly lled with hydraulic lluid under pressure since the pump 28 delivers a substantially higher volume than does the pump 26. After the pressure in the system reaches the predetermined value-about 500 p. s. i.-for which the pressure relief or unloading valve 72 is set, the latter opens .and the pump 28 thereafter circulates iluid to the tank and the valve 52 closes. The high pressure pump 26 continues to pump uid under pressure through the conduit 42 until the pressure in that line reaches a second and higher predetermined value, which will be sulicient to cause the inatable bag 24 to complete the pressing of the rubber mat 22 about the workpieces 16 and finish forming them against the dies 20. At this pressure value, either of the switches 148 is actuated to stop the motor 38.

At the instant the motor is stopped the compressed oil or hydraulic uid in the bag 24 begins to expand, thereby elfecting decompression of the confined fluid. The oil or uid expands through the conduits 42 and 38 to the high pressure pump 26 to drive the latter in the reverse direction, and iluid is expelled through the port 34, conduits 64 and 92, valve 90L, conduit 100, valve 90R and conduit 102, to the tank. The -actuation of the switches 148 also closes a timer circuit, and after a very short and predetermined interval, for example one-tenth of a second, the timer closes a circuit to the motor 30 so that the latter will run in the same direction as the pump 26 to limit the speed of rotation of the entire unit consisting of the motor 30 and pumps 26 and 28 thereby to control the rate of decompression.

After the pressure has dropped to a value about or slightly under that at which the valve 72 was opened, the latter closes and the low pressure pump takes in uid from the bag 24 through the valve 52, which was opened by uid pressure from the pump 26, and port 45 and delivers it through the port 46 and conduit 47 to the tank 31. The pumps 26 and 28 act in concert to complete the evacuation of the bag 24, and when it has been completely emptied, the pump 28 creates the partial vacuum as previously described. When the pressure in the system reaches zero p. s. i. the check valve 86 opens and the pump 26 takes in lluid from the tank 31 through the port 32. This vacuum is communicated to the vacuum switch 122 and upon reaching a predetermined value closes the switch to energize the solenoid 114L to permit the pump 26 to supply hydraulic fluid under pressure to the rod end of the hydraulic motor SSL and thereby to Withdraw the work table 18 from the housing 14. At the end of the table travel, a limit switch in the solenoid circuit is actuated to open the circuit and stop the table movement. However, the unit of the motor 30 and the two pumps 26 and 28 continues to drive in the same direction and the vacuum is maintained on the bag so that the system is in readiness for a subsequent work forming operation.

Fig. 3 illustrates a modied hydraulic circuit embodying lall of the operating principles of the circuit of Fig. l and which is particularly useful for those presses hav ing high power requirements and a bag with a substantially higher volume than that usually incorporated in the press circuit of Fig. 1. For example the bag in a small press may measure 20 inches by 50 or 72 inches While 'a very large press might have a bag measuring 50 inches by 168 inches. The power for the small press can be obtained from a 25 H. P. motor While a large press will take a power plant of H. P. Of course the total volume of uid pumped in the larger presses is substantially greater than that for a small press and the working pressures are sometimes required to be higher. But it is desirable to maintain the work cycle time as short as possible so that the press output of the larger presses is high. The hydraulic circuit of Fig. 3 is more flexible in its applications than that of Fig. l and is adaptable to.

presses of varying sizes and pump and power requirements.

In Fig. 3 the same reference characters, sometimes with a suix letter, are used :to designate the same or similar circuit components as are embodied in the circuit of Fig. l.

A pair of low pressure, high volume pumps 28A and 28B are connected lto opposite ends of the shaft of a motor 3.0L. Ports 45A and 45B of these pumps are connected to Ithe prell valve 52 which is connected by the conduits 54 and 42 to the bag 24. A pair ofunloading Valves 72A and 72B are connected to the pump ports 45A and 45B to dump the output of the low pressure pumps to the tank 31 when the system pressure reaches a value of about 500 p. s. i. thereby to terminate the uid transferring action of these pumps. In addition a pressure switch 152, connected to conduit 42, is set to stop the motor 30L when the l:system pressure reaches 600 p. s. i. during bag inflation and work forming and to restart it when this pressure falls to 400 p. s. i. in the decompression and evacuation phases of the press cycle. When the pumps 28A and 28B are maintaining a vacuum on the bag 7 24 they suck in uid through. check valves 82A and 82B in the conduits 80A and 80B; respectively.

Fluid under high pressure is lsupplied from a high pressure, low volume pump 26A driven by a motor 30H which is connected to the conduit 42by a conduit 38 leading from pump port 32A and having a branch conduit 33A connected to the pressure relief valve 40.

This pump also furnishes fluid to open the prell valve 52 and to operate the table motors SSL and SSR. Table movement is controlled by a pair of hydraulically operated solenoid controlled four way valves 90LA and 9011A, the port arrangements and connections for which are the same as those for the valves 901. and 90R of the first embodiment. Hydraulically operated valves are used in this press circuit because. of the higher operating pressures, and fluid for operating the valves is supplied through a pilot conduit system 154 connected to conduit 64 and pump port 34a. Fluid for the hydraulic motors is supplied through a pair of pressure relief valves 156 and S connected in :series by a conduit 160, to the valve 90LA by conduit 92A and from the conduit 64 and pump port 34A by a conduit 162 having a pressure producing restriction 94A therein. The restriction insures that adequate pressure, for example 150 p. s. i., is provided to open the prefill valve 52 and to operate the four-way valves and table motors, particularly at the outset of the press cycle when the pump 26A is otherwise idling.

Pressure relief valve 156 is normally yset to open at about 1500 p. s. i. so thatback pressure between 1500 p. s. i. and 2000 p. s. i. will be available to open the prell valve at a system pressure of 400 p. s. i. during decompression and evacuation of the circuit. When the system pressure has decreased to 50 p. S. i. a pilot operated, spring return four-way valve 164 having a pilot port 166 connected to conduit 38 by a conduit 168 and a port 170 connected by a conduit 172 to pilot connection on relief valve 156, will open the relief valve 156 thus reducing the pilot pressure in the system to that at the orifice 94A, namely 150 p. s. i. The construction of the valve 164 is the same as that of the valve 124.

The relief valve 158 is set at 750 p. s. i. to protect the pump and system against overload during table movement, and it vents to the tank through the conduit 120.

The operation of the circuit of Fig. 3 differs only slightly from that of Fig. l, but its operating principle is the same. When the press is started after being idle the motor BGL is started first and drives the pumps 28A and 28B in vacuum creating direction. Immediately thereafter the motor H ystarts automatically to drive the pump 26A in pilot pressure producing direction, opening the valve 52 and circulating fluid through the valves 90LA and 90RA.

At the proper time a work table 18 is moved into work forming position, and both motors are stopped with motor SGL being started in the bag inating direction an instant before the motor 30H. All pumps operate to inflate the bag and increase the pressure.

When the system pressure reaches approximately 500 p. s. i. the unloading valves 72A and 72B are opened and the output from the low pressure pumps is diverted to the tank 31, the valve 52, of course, closing. At 600 p. s. i. system pressure the pressure switch 152 cuts off current to the motor 30L which stops.

The pressing phase of the cycle is completed, and decompression and evacuation are initially carried out by the pump 26A braked by the motor 30H in the manner previously described. Pilot pressure in the conduit 66 and at the prell valve 52 will be between 1500 p. s. i. and 2000 p. s. i. from the restrictive effect of the relief valve 156. When the systemv pressure nears 400 p. s. i. during evacuation the valve 52 will be opened-by the pilot pressure, and the pressure switch 152 will energize the circuit to the motor 30L to start it driving the pumps 28A and 28B in bag evacuating direction.

At p. s. i. system pressure the valve 1'56 will openA 8 and the-pilot pressure will drop: to 150 p. s. i. The press cycle is then completed as-previously described.

The capacity of the bag24l and the work forming pressure desired dictate the number of pumps used and their specific arrangement. For example four pumps could be used, two each of the low and high pressure outputs. It is also to be understood that the operating pressures given inthe foregoing description are merely illustrative, as the actual working pressure will depend upon specific press and work forming characteristics and conditions. However, when more than one motor is used to drive the pumps it irs desirable that they be started sequentially rather than simultaneously to prevent line overload.

The illustrated systems are designed for two work tables 18 to be introduced into the cylindrical housing 14 from opposite ends; and this is desirable so that one table can be readied for a work operation while work is being performed on pieces on the other table.

It will be observed from the foregoing description that the objectives which are claimed for this invention at the outset of the specification are fully attained.

While preferred embodiments of the anti-shock hydraulic circuit constituting this invention have been shown and. described, it will be apparent that numerous modifications and variations thereof may be made without departing from the underlying principles of the invention. It is, therefore, desired by the following claims to include Within the scope of the invention all such variations and modifications by which substantially the results of this invention may be obtained through the use of the same or equivalent means.

What isv claimed as new and desired to be secured by United States Letters Patent is:

l. In a hydraulic system, means forming an enclosed work space adapted to receive hydraulic fluid under pressure and to have the uid evacuated therefrom, a fluid reservoir, a high pressure pump having a pump port connected to said work space, a low pressure high volume pump having a pump port connected to said work space, a normally closed valve through which said low pressure pump supplies fluid to said work space, said low pressure pumphaving a second port connected to said reservoir, said second port from said high pressure pump being connected to said valve to move it to open position when said high pressure pump is delivering uid through said second port, an unloading valve connected to said rst port from said low pressure pump and connected thereto upstream of said normally closed valve and adapted to divert the output from said low pressure pump to the reservoir when the pressure in the system reaches a predetermined value, a reversible motor connected to drive both pumps simultaneously, means for stopping said motor when the pressure in the system reaches a second predetermined value higher than said iirst predetermined pressure value and for reversing said motor after a short time interval whereby the hydraulic fluid under pressure in said work space drives said high pressure pump in the reverse direction before said motor does so and effects decompression of the fluid under pressure in said work space, said pumps coacting to evacuate said work space of fluid, and said low pressure pump thereafter forming a partial vacuum in said work space.

2. ln a hydraulic system, means forming an enclosed expansible work space adapted to receive hydraulic fluid under pressure and to have the fluid evacuated therefrom, a Huid reservoir, a pair of pumps connected to transfer fluid between said work space and said reservoir with the liuid being supplied to said work space under pressure, a normally closed valve interposed between one of said pumps and said work space, the second of said pumps being connected directly to said work space, means for diverting the output from said one pump to said reservoir when the pressure in the system reaches a predetermined value, a reversible motor connected to drive both pumps t simultaneously, means for stopping said motor when the pressure in the system reaches a second predetermined value higher than said first predetermined pressure value and for starting said motor in the reverse direction after a short time interval whereby the hydraulic fluid under pressure in said work space drives said second pump in the reverse direction before said motor does to effect decompression of the iluid under pressure in said work space, said pumps thereafter coacting to evacuate said work space of iluid.

3. ln a hydraulic system, means forming an enclosed expansible work space adapted to receive hydraulic fluid under pressure and to have the fluid evacuated therefrom, a iluid reservoir, a pair of pumps connected to transfer iluid between said work space and said reservoir with the iluid being supplied to said work space under pressure, a normally closed Valve interposed between one of said pumps and said work space, the second of said pumps being connected directly to said work space, an unloading valve connected to said one pump upstream of said normally closed valve and adapted to divert the outlet from said one pump to said reservoir when the pressure in the system reaches a predetermined value, a reversible motor connected to drive both pumps simultaneously, means for stopping said motor when the pressure in the system reaches a second predetermined value higher than said iirst predetermined pressure value and for starting said motor in the reverse direction after a short time interval whereby the hydraulic iluid under pressure in said work space drives said second pump in the reverse direction before said motor does so to effect decompression of the fluid under pressure in said work space, said pumps thereafter coacting to evacuate said Work space of iluid, and said one pump forms a partial vacuum in said work space.

4. in a hydraulic system, means forming an enclosed expansible work space adapted to receive hydraulic lluid under pressure and to have the fluid evacuated therefrom, a fluid reservoir, a pair of pumps connected to transfer fluid between said work space and said reservoir with the iluid being supplied to said work space under pressure, a normally closed valve interposed between one of said pumps and said work space, the second of said pumps being connected directly to said work space and to said valve to move it to open position when said pumps are operating to transfer fluid from said work space to said reservoir, a reversible motor connected to drive both pumps simultaneously, means for stopping said motor when the pressure in the system reaches a predetermined value and for starting said motor in the reverse direction after a short time interval whereby the hydraulic fluid under pressure in said work space drives said second pump in the reverse direction before said motor does so to effect decompression of the iluid under pressure in said work space, said pumps coacting to evacuate said work space of fluid, and said one pump forming a partial vacuum in said work space.

5. In a hydraulic system, means forming an enclosed expansible work space adapted to receive hydraulic fluid under pressure and to have the fluid evacuated therefrom, a fluid reservoir, a pair of pumps connected to transfer fluid between said work space and said reservoir with the lluid being supplied to said work space under pressure, a normally closed valve interposed between one of said pumps and said work space, the second of said pumps being connected directly to said Work space and to said valve to move it to open position when said pumps are operating to transfer fluid from said Work space to said reservoir, means for diverting the output from said one pump to said reservoir when the pressure in the system reaches a predetermined value while said pumps are operating to supply luid to said work space, a reversible motor connected to drive both pumps simultaneously, means for stopping said motor when the pressure in the system reaches a second predetermined value higher than said first predetermined pressure value and for starting said motor in the reverse direction after a short time interval whereby the hydraulic iluid under pressure in said work space drives said second pump in the reverse direction before said motor does so to effect decompression of the fluid under pressure in said work space, said pumps coacting to evacuate said work space of tluid, and said one pump forming a partial vacuum in said work space.

6. In a hydraulic system, means forming an enclosed expansible work space adapted to receive hydraulic iluid under pressure and to have the fluid evacuated therefrom, a fluid reservoir, a high pressure pump and a low pressure pump connected to transfer fluid between said work space and said reservoir with the iluid being supplied to said work space under pressure, a normally closed valve interposed between said low pressure pump and said work space, said high pressure pump being connected directly to said work space and to said valve to move it to open position when said high pressure pump is operating to transfer fluid from said work space to said reservoir, means for diverting the output from said low pressure pump to said reservoir when the pressure in the system reaches a predetermined value, a reversible motor connected to drive both pumps simultaneously, means for stopping said motor when the pressure in the system reaches a second predetermined value higher than said rst predetermined pressure value and for starting said motor in the reverse direction after a short time interval whereby the hydraulic iluid under pressure in said work space drives said high pressure pump in the reverse direction before said motor does so to effect decompression of the iluid under pressure in said work space, said pumps coacting to evacuate said work space of fluid, and said low pressure pump forming a partial vacuum in said work space.

7. In a hydraulic system, means forming an enclosed expansible work space adapted to receive hydraulic fluid under pressure and to have the iluid evacuated therefrom, a fluid reservoir, a high pressure pump and a low pressure pump connected to transfer iluid between said work space and said reservoir with the iluid being supplied to said work space under pressure, a normally closed valve interposed between said low pressure pump and said work space, said high pressure pump being connected directly to said work space and to said valve to move it to open position when said high pressure pump is operating to transfer iluid from said work space to said reservoir, an unloading valve connected to said low pressure pump upstream of said normally closed valve and adapted to divert the output from said low pressure pump to the reservoir when the pressure in the system reaches a predetermined value, a reversible motor connected to drive both pumps simultaneously, means for stopping said motor when the pressure in the system reaches a second predetermined value higher than said first predetermined pressure value and for starting said motor in the reverse direction after a short time interval whereby the hydraulic lluid under pressure in said work space drives said high pressure pump in the reverse direction before said motor does so to effect decompression of the lluid under pressure in said work space, said pumps coacting to evacuate said work space of lluid, and said low pressure pump forming a partial vacuum in said work space.

8. In a hydraulic system, means forming an enclosed expansible work space adapted to receive hydraulic fluid under pressure and to have the fluid evacuated therefrom, a iluid reservoir, a plurality of pumping means connected to transfer iluid between said work space and said reservoir with the iluid supplied to said work space under pressure, a normally closed Valve interposed between one of said pumping means and said work space, the pumping means being connected directly to said work space and to said valve when said pumping means are operating to transfer fluid from said work space to said 11 reservoir, reversible power means connected to drive said pumping means, meansffor stopping said power means when the pressure in the systemk reaches a predetermined value and for starting said power means driving in the reverse direction after a short time interval whereby the hydraulic fluid in said work space drives some of said pumping means in the reverse direction before said power means does so to effect decompression of the fluid under pressure in said work space, said pumping means coacting to evacuate said work space of fluid, and aportion of said pumping means forming a partial vacuum in said work space.

9. In a hydraulic system, means forming an enclosed expansible work space adapted to receive hydraulic ftuid under pressure and to have the fluid evacuated therefrom, a fluid reservoir, a high pressure pumping means and a low pressure pumping means connected to transfer uid between said reservoir and said work space, the uid being transferred to said work space under pressure, a normally closed valve interposed between said work space and said low pressure pumping means, said high pressure pumping means being connected directly to said work space, means connecting said high pressure pumping means to said valve and operative to open said valve when said high pressure pumping means is operating in work space evacuating direction, means for rendering said low pressure pumping means ineffective to transfer fluid when the system pressure reaches a predetermined value so that said valve closes and continued pumping to increase pressure in the system is effected only by said high pressure pumping means, and means for stopping the pumping of said high pressure pumping means when the system pressure reaches a second and higher predetermined value, and for starting said high pressure pumping means in the opposite direction after a short time interval, whereby decompression of the fluid in said work space is effected by expansion and ow of fluid through said high pressure pumping means to drive it in the reverse direction during said time interval.

10. In a hydraulic system, means forming an enclosed expansible work space adapted to receive hydraulic fluid under pressure and to have the iluid evacuated therefrom, a fluid reservoir, a pair of pumping means connected to transfer uid between said reservoir and said work space, the fluid being transferred to said Work space under pressure, a normally closed valve interposed between said work space and one of said pumping means, said other pumping means being connected directly to said work space, means connecting said other pumping means to said valve and operative to open said valve when said other pumping means is operating in work space evacuating direction, means for rendering said one pumping means ineffective to transfer fluid when the system pressure is higher than a rst predetermined value so that said valve closes and continued pumping to increase pressure in the system is effected only by said other pumping means, and means for stopping the pumping of said other pumping means when the system pressure reaches a second and higher predetermined value and for starting said other pumping means in the opposite direction after a short time interval7 whereby decompression of the uid in said work space is effected by expansion and tlow of fluid through said other pumping means initially to drive it in said opposite direction, and both said pumping means coacting to evacuate said'work space when the system pressure falls to and below said rst predetermined value.

ll. in a hydraulic system, means forming an enclosed cxpansible work space adapted to receive hydraulic fluid under pressure and to have the fluid evacuated therefrom, a fluid reservoir, a high pressure pumping means and a low pressure pumping means -connected to transfer fluid between said reservoir and said. work space, the fluid being transferred to said work space under pressure, `a normall'y closed valve interposed `between said work space and said low pressure pumping means, said high pressure pumping means being connected directly to said work space, means connecting said high pressure pumping means to said valve and operative to open said valve when said high pressure pumping means is operating in work space evacuating direction, means for rendering said low pressure pumping means ineffective to transfer uid when the system pressure is higher than a first predetermined value so that said valve closes `and continued pumping to increase pres in the system is effected only by said high pressure pumping means, and means for stopping the pumping action of said high pressure pumping means when the system pressure reaches a second and higher predetermined value and for starting said high pressure pumping means in the opposite direction after a short time interval, whereby decompression of the uid in said work spa-ce is effected by expansion and flow of fluid through said high pressure pumping means initially to drive it in said opposite direction, both said pumping means coacting to evacuate said work space when the system pressure falls to and below said first predetermined value, and said low pressure pumping means thereafter forming a partial vacuum in said work space.

12. In a hydraulic system, means forming an enclosed expansible work space adapted to receive hydraulic fluid under pressure and to have the uid evacuated therefrom, a uid reservoir, a high pressure pump and a low pressure pump adapted to transfer fluid between said reservoir and said work space, the fluid being transferred to said work space under pressure, a reversible power means for and connected to drive each of said pumps, a normally closed valve interposed between said work space and said low pressure pump, said high pressure pump being connected directly to said work space, means connecting said high pressure pump to said' valve and operative to open said valve when said high pressure pump is driven in work spa-ce evacuating direction, an unloading valve connected between said low pressure pump and said normally closed valve and adapted' to divert the output from said low pressure pump to said reservoir when the pressure in the system reaches a rst predetermined value, means for stopping said low pressure pump power means and restarting it in the reverse direction at pressures slightly higher than and slightly lower than said first predetermined pressure, respectively, and means for stopping said high pressure pump power means when the system pressure reaches a second and higher predetermined valueand for restarting it in the opposite direction after a short time interval, whereby decompression of the duid in said work space is effected by expansion and' ow of fluid through said high pressure pump initially to drive it in said opposite direction during said time interval and said high pressure power means thereafter controls the rate of decompression.

13. in a hydraulic system, means forming an enclosed expansible work space adapted to receive hydraulic fluid under pressure and to have the fluid evacuated therefrom, to place said work space under -a partial vacuum, a fluid reservoir, a high pressure pump and a low pressure pump adapted to transfer fluid between said reservoir and said work space, the uid being transferred'to said work space under pressure, a reversible power means for 4and connected to drive each of said pumps, a normally closed valve interposed Ibetween said work space and said low pressure pump and opened by the latter when driven in such direction as to till said work space, said high pressure pump being connected directly to said work space, means connecting said high pressure pump to said valve and operative to open saidvalve when said high pressure pump is driven in work space evacuating direction and the system pressure falls below a first predetermined value, an unloading valve connected between said low pressure pump and said normally closed valve and ladapted to divert the output from said low pressure pump to said reservoir when the pressure in the system rises tosaid rst predetermined pressure, means for stopping said low pressure pump power 13 means and restarting it in the reverse direction at pressures slightly higher than and slightly lower than said rst predetermined pressure, respectively, and means for stopping said high pressure pump power means when the system pressure reaches a second yand higher predetermined value and for restarting it in the opposite direction after a short time interval, whereby decompression of the fluid in said work space is effected by expansion and ilow of uid through said high pressure pump initially to drive it in said opposite direction during lsaid time interval, said 10 2 644 03 i4 high pressure power means thereafter controls the rate of decompression, `both said pumps coacting to evacuate said Work space when the system pressure falls to and below said iirst predetermined value, and said low pressure pump creates and holds a partial vacuum in said work space.

References Cited in the file of this patent UNITED STATES PATENTS Rohrscheib et al July 7, 1953 

